Windows NT Replies to Address Mask Requests

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Article ID: 186463 - View products that this article applies to.
This article was previously published under Q186463
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IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.
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Your computers running Windows NT may be responding to Address Mask requests on the network, which could enable malicious users to discover some of your network topology information.


To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
152734 How to Obtain the Latest Windows NT 4.0 Service Pack

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

  1. Start Registry Editor (Regedt32.exe).
  2. Go to the following subkey:
  3. Click Add Value on the Edit menu.
  4. Create a new value using the following information:
    Value Name: EnableAddrMaskReply Data Type : Reg_DWORD Data : 1 (default=
  5. Click OK, close Registry Editor, and restart your computer.


Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.


RFC 1122, section

A system MUST NOT send an Address Mask Reply unless it is an
authoritative agent for address masks. An authoritative agent
may be a host or a gateway, but it MUST be explicitly configured
as an address mask agent. ...

RFCs may be obtained through the Internet as follows:

Paper copies of all RFCs are available from the NIC, either individually or on a subscription basis (for more information contact NIC@NIC.DDN.MIL). Online copies are available through FTP or Kermit from NIC.DDN.MIL as rfc/rfc####.txt or rfc/rfc####.PS (#### is the RFC number without leading zeros).

Additionally, RFCs may be requested through electronic mail from the automated NIC mail server by sending a message to SERVICE@NIC.DDN.MIL with a subject line of "rfc ####" for text versions, or a subject line of "rfc ####.PS" for PostScript versions. To obtain the RFC index, the subject line of your message should read "rfc index".


Article ID: 186463 - Last Review: October 8, 2013 - Revision: 1.3
  • Microsoft Windows NT Server 4.0, Terminal Server Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows NT Workstation 4.0 Developer Edition
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